mhi: core: do not assert device wake when processing completion event

When processing completion events, we do not need to assert
wake because device has already completed required work.

CRs-Fixed: 2248650
Change-Id: I0a1981baaae2784a23abf1fdc262bc9e7f2aa460
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>